1. Field of the Invention
This invention is concerned with analysis of organic additives and contaminants in plating baths as a means of providing control over the deposit properties.
2. Description of the Related Art
Electroplating baths typically contain organic additives whose concentrations must be closely controlled in the low parts per million range in order to attain the desired deposit properties and morphology. One of the key functions of such additives is to level the deposit by suppressing the electrodeposition rate at protruding areas in the substrate surface and/or by accelerating the electrodeposition rate in recessed areas. Accelerated deposition may result from mass-transport-limited depletion of a suppressor additive species that is rapidly consumed in the electrodeposition process, or from accumulation of an accelerating species that is consumed with low efficiency. The most sensitive methods available for detecting leveling additives in plating baths involve electrochemical measurement of the metal electrodeposition rate under controlled hydrodynamic conditions for which the additive concentration in the vicinity of the electrode surface is well-defined.
Cyclic voltammetric stripping (CVS) analysis [D. Tench and C. Ogden, J. Electrochem. Soc. 125, 194 (1978)] is the most widely used bath additive control method and involves cycling the potential of an inert electrode (e.g., Pt) in a plating solution between fixed potential limits so that metal is alternately plated on and stripped from the electrode surface. Such potential cycling is designed to establish a steady state for the electrode surface so that reproducible results are obtained. Accumulation of organic films or other contaminants on the electrode surface can be avoided by periodically cycling the potential of the electrode in a plating solution without organic additives and, if necessary, polishing the electrode using a fine abrasive. Cyclic pulse voltammetric stripping (CPVS), also called cyclic step voltammetric stripping (CSVS), is a variation of the CVS method that employs discrete changes in potential during the analysis to condition the electrode so as to improve the measurement precision [D. Tench and J. White, J. Electrochem. Soc. 132, 831 (1985)]. A rotating disk electrode configuration is typically employed for both CVS and CPVS analysis to provide controlled hydrodynamic conditions.
For CVS and CPVS analyses, the metal deposition rate may be determined from the current or charge passed during metal electrodeposition but it is usually advantageous to measure the charge associated with anodic stripping of the metal from the electrode. A typical CVS/CPVS rate parameter is the stripping peak area (Ar) for a predetermined electrode rotation rate. The CVS method was first applied to control copper pyrophosphate baths (U.S. Pat. No. 4,132,605 to Tench and Ogden) but has since been adapted for control of a variety of other plating systems, including the acid copper sulfate baths that are widely used by the electronics industry [e.g., R. Haak, C. Ogden and D. Tench, Plating Surf. Fin. 68(4), 52 (1981) and Plating Surf. Fin. 69(3), 62 (1982)].
Acid copper sulfate electroplating baths require a minimum of two types of organic additives to provide deposits with satisfactory properties and good leveling characteristics. The suppressor additive (also called the “polymer”, “carrier”, or “wetter”, depending on the bath supplier) is typically a polymeric organic species, e.g., high molecular weight polyethylene glycol or polypropylene glycol, which adsorbs strongly on the copper cathode surface to form a film that sharply increases the overpotential for copper deposition. This prevents uncontrolled copper plating that would result in powdery or nodular deposits. An anti-suppressor additive (also called the “brightener”, “accelerator” or simply the “additive”, depending on the bath supplier) is required to counter the suppressive effect of the suppressor and provide the accelerated deposition within substrate recesses needed for leveling. Plating bath vendors typically provide additive solutions that may contain additives of more than one type, as well as other organic and inorganic addition agents. The suppressor additive may be comprised of more than one chemical species and generally involves a range of molecular weights.
Acid copper sulfate baths have functioned well for plating the relatively large surface pads, through-holes and vias found on printed wiring boards (PWB's) and are currently being adapted for plating fine trenches and vias in dielectric material on semiconductor chips. The electronics industry is transitioning from aluminum to copper as the basic metallization for semiconductor integrated circuits (IC's) in order to increase device switching speed and enhance electromigration resistance. The leading technology for fabricating copper IC chips is the “Damascene” process (see, e.g., P. C. Andricacos, Electrochem. Soc. Interface, Spring 1999, p. 32; U.S. Pat. No. 4,789,648 to Chow et al.; U.S. Pat. No. 5,209,817 to Ahmad et al.), which depends on copper electroplating to provide complete filling of the fine features involved. The organic additives in the bath must be closely controlled since they provide the copper deposition rate differential required for bottom-up filling.
As the feature size for the Damascene process has shrunk below 0.2 μm, it has become necessary to utilize a third organic additive in the acid copper bath in order to avoid overplating the trenches and vias. Note that excess copper on Damascene plated wafers is typically removed by chemical mechanical polishing (CMP) but the copper layer must be uniform for the CMP process to be effective. The third additive is called the “leveler” (or “booster”, depending on the bath supplier) and is typically an organic compound containing nitrogen or oxygen that also tends to decrease the copper plating rate. In order to attain good bottom-up filling and avoid overplating of ultra-fine chip features, the concentrations of all three additives must be accurately analyzed and controlled.
The suppressor, anti-suppressor and leveler concentrations in acid copper sulfate baths can all be determined by CVS analysis methods based on the effects that these additives exert on the copper electrodeposition rate. At the additive concentrations typically employed, the effect of the suppressor in reducing the copper deposition rate is usually much stronger than that of the leveler so that the concentration of the suppressor can be determined by the usual CVS response curve or dilution titration analysis [W. O. Freitag, C. Ogden, D. Tench and J. White, Plating Surf. Fin. 70(10), 55 (1983)]. Likewise, the anti-suppressor concentration can be determined by the linear approximation technique (LAT) or modified linear approximation technique (MLAT) described by R. Gluzman [Proc. 70th Am. Electroplaters Soc. Tech. Conf., Sur/Fin, Indianapolis, Ind. (June 1983)]. A method for measuring the leveler concentration in the presence of interference from both the suppressor and anti-suppressor is described in U.S. Pat. No. 6,572,753 to Chalyt et al.
CVS dilution titration analysis for the suppressor additive typically involves measurements of Ar(0) for the plating bath supporting electrolyte (without organic additives) and Ar values for a plurality of measurement solutions resulting from standard addition of the plating bath to the supporting electrolyte. The Ar parameter may be used directly for the dilution titration analysis but use of the normalized Ar/Ar(0) parameter tends to minimize measurement errors associated with changes in the electrode surface, background bath composition, and temperature. The suppressor concentration in the plating bath is determined from the volume fraction of plating bath required to decrease Ar/Ar(0) or Ar to a predetermined endpoint value, which may be a numerical value or a minimum value corresponding to substantially maximum suppression. The suppressor concentration in the plating bath is calculated by reference to the concentration of suppressor required to attain the Ar/Ar(0) or Ar titration endpoint value in a calibration run involving standard additions of the suppressor additive to the supporting electrolyte. In the alternative response curve analysis, the suppressor concentration is determined from the Ar/Ar(0) or Ar value for a measurement solution (supporting electrolyte plus a known volume fraction of plating bath sample) by interpolation with respect to an appropriate calibration curve. The effects of the anti-suppressor and leveler additives on the suppressor analysis are typically small but can be taken into account by utilizing a supporting electrolyte containing the concentrations of these additives measured or estimated to be present in the plating bath being analyzed.
For analysis of the anti-suppressor additive by the modified linear approximation technique (MLAT), the CVS rate parameter Ar is first measured in a supporting electrolyte containing no anti-suppressor but with a sufficient amount of suppressor species added to substantially saturate suppression of the copper deposition rate. A predetermined volume ratio of the plating bath sample to be analyzed is then added to this fully-suppressed supporting electrolyte and Ar is again measured. The Ar measurement is then repeated in this measurement solution (supporting electrolyte plus a known volume fraction of the plating bath) after each addition (typically two) of known amounts of the anti-suppressor additive only. The concentration of the anti-suppressor in the plating bath sample is calculated assuming that Ar varies linearly with anti-suppressor concentration, which is verified if the change in Ar produced by standard additions of the same amount of anti-suppressor are equivalent.
In practice, the suppressor and anti-suppressor analyses are performed using the same electrochemical cell, and it has generally been deemed necessary to thoroughly clean and rinse the cell between these analyses to avoid cross-contamination errors. Residual organic additive species are a particular cross-contamination concern since such species strongly affect copper electrodeposition rate measurements, which are the basis for all CVS analyses. The cleaning and rinsing operations consume substantial amounts of both time and expensive chemicals, and generate significant quantities of hazardous wastes. These are especially serious issues for Damascene plating bath analyses, which must be performed frequently to attain consistent bottom-up filling and high product yields.